Dude, I had my betta in dorm last year and trust me lol... far easier to maintain a 5g tank once a week than a .5g tank twice a day.
I strongly suggest you prowl craigslist... so many noobs who put a goldfish in a 5g, get frustrated when it dies in a week and sell all the equipment off on CL for a fraction of the price. Happens every day.
If taken care of well, your betta could easily be with you for all 4 years of college... sometimes the live up to 7! But I honestly would not count on your betta being around for much longer in that .5g... the ammonia produced by his waste is in such a high concentration. You may recognize ammonia as a cleaning agent, so obviously your fish cannot survive for too long in this stuff!
Here is a minibow 5 on petco.com for $45 with free shipping... that's like two tanks of gas or two 24 packs of beer! Or the cost of eating out 5 times. Or a new shirt.
The minibow 2.5 is also on sale, but you can get twice the tank for ten bucks more so the 5g is more worth the money. IMO a decent aquarium is well worth the money... when your fish actually has sufficient swimming space they are really relaxing to watch and it's really fun to slowly add decorations to the tank and get it looking really good.
I started with a betta in a 1g dorm room bowl and it has evolved into a really fun, educational, relaxing hobby for me.